Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Resistance Help

  1. Jul 6, 2011 #1

    I had to measure the resistance between different sets of bridge deck reinforcement steel bars using a Wavetek HD110T multimeter and a wire. The values varied between 1.9 to 2.8 ohms. When I measured the resistance of the wire itself, it came out to be 2.1 ohm.

    I could not understand how the resistance of wire itself is more than some of the resistance measurements taken between steel bars. If it is a common issue, can you please explain the justification? I have not much background in electrical engineering.

    Thank you.
  2. jcsd
  3. Jul 6, 2011 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    The contact resistance was probably variable enough to cause the measurement uncertainties. To get more accurate measurements, you would need to come up with some very consistent way of making the contacts, not just holding them together by hand.

    You could use screw contacts with star washers, for example, and use a consistent torque when tightening the nut. If you do something like that, you should get numbers that make more sense.

    How accurate do you need the resistance measurements? There are some applications where the accuracy of the measurement is very important (like in detecting poorly-crimpted connectors in cable assemblies....)
  4. Jul 6, 2011 #3
    Thank you, Sir, for your timely help. Fortunately, I do not need more accuracy in the measurements. I am putting together a journal article about our field corrosion testing of a bridge deck, so thought of clearing my doubt about this resistance issue.

    I used a vice grip to attach the wire to the reinforcement steel bars. But I wouldn't be too confident about the contact, since the bars had some corrosion byproducts on their surfaces.
  5. Jul 6, 2011 #4


    User Avatar

    Staff: Mentor

    Can you clean the surface first, or does that not help the overall measurements?
  6. Jul 6, 2011 #5
    We drill and remove only about 2 in. diameter of concrete cover and the bar is at least 2 in. deep. Since the reinforcement is hard to access, we tap a thin steel bar into the reinforcement for convenient connections. So, cleaning is always not perfect.
  7. Sep 3, 2011 #6
    I have to calculate the dc resistance of 1.5 mm2 flexible copper wire . How i can calculate?
  8. Sep 3, 2011 #7
    [tex]R=\frac l {\sigma_{Cu}a}\;\hbox { where } \;a=1.5mm^2, l \;\hbox { in mm and }\;\sigma_{Cu}=5.8\times 10^7[/tex]
  9. Sep 3, 2011 #8
    The Wavetek HD110T will not make accurate measurements under the conditions described.
    A good 4 wire Kelvin ohmmeter is required.
    Google" 4 wire resistance measurements" for theory of how 4 wire measurements are made.
  10. Sep 3, 2011 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Ordinary multimeters measure resistance by injecting a small current usually a milliamp or less, reading the voltage and calculating the result by ohm's law. I once had a general purpose industrial meter not unlike yours that injected two milliamps.

    IF there is any electrochemical stuff going on, which seems likely in concrete under a bridge where it's apt to be moist , a miniature battery effect - dissimilar metals in presence of electrolyte - can result and that'll make a few millivolts.

    The meter has no idea whether the millivolts it finds there belong to a micro-battery effect or to the honest ohms of resistance you're after.

    Here's something you might try:

    1. after connecting the meter but before taking your reading , switch the meter to lowest DC volt scale (probably 200mv?) and record reading.
    If you read a few millivolts you probably have moisture someplace making a microbattery.
    That's no big deal, just be aware it'll offset your ohm reading.

    2. Then take your reading twice, swapping meter leads between, so as to get readings with meter sending its current in both directions.

    Probably the average of the two readings is closer than either one.
    That's because the microbattery raised one reading and lowered the other.

    and i'd bet lunch the difference in your ohm readings correlates with whatever DC voltage you measured in step 1.

    old jim
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook